Added explicit discussion of several encrypted env var things that confused me

I was working with encrypted env vars yesterday, and I was confused by several things.

* I didn't realize you needed to pass the env var name to travis encrypt.
* I wasn't sure if the key should be "secure" for all vars, or if the example was just using the name "secure."
* I also wasn't sure if you could put multiple "secure" entries in one .travis.yml.

Updated the page to say these things explicitly.  Hope it helps the next person.

Thank you for creating and maintaining travis!

Mike